Parking meters have evolved into rather sophisticated devices as compared with meters of the past. The demands on parking meter manufacturers to provide increased functionality at reduced cost are becoming increasingly more severe. Different jurisdictions have different needs and requirements. Parking meters must be capable of displaying messages in the language required by the customer. To avoid the need of having a number of different models and the associated costs of doing so, a parking meter must be configurable to allow the language of messages to be changed easily. Some jurisdictions require the use of coins while others require the use of so called “smart cards” as the form of payment for parking time. Some jurisdictions require that the parking meter be capable of being interrogated electromagnetically or optically. If a meter is to be capable of being used in different countries, the meter must be capable of discriminating from coins of several countries. Some jurisdictions require that rates for parking time change at period intervals.
Most parking meters now available are electronic and, therefore, require a source of power in the form of batteries. The most severe requirement imposed by customers may be keeping energy consumption by parking meter electronics to a minimum. Obviously, in order to reduce the cost of replacement batteries and the costs associated with physically replacing batteries, an important requirement imposed by customers is maximum battery life. These and other such requirements are over and above the basic functional requirements of parking meters which are to reliably detect the presence of coins, identify the coin, dispense the appropriate amount of time purchased and accurately provide the amount of time purchased before a time expired message.
To be competitive, a parking meter manufacturer must be able to offer a parking meter having these and other, sometimes unpredictable, functions.
Coin Detection
Electronic parking meters typically include a coin proximity detector to signal a microprocessor when a coin enters in a coin chute. The classic inductive proximity detector for metal objects consists of an inductive sensor, an oscillator and a detector circuit. The oscillator and sensor generate an electromagnetic field which radiates and which is often directed toward the target. When a metal object enters the electromagnetic field, eddy currents are induced into the surface of the object resulting in a loading effect which reduces the amplitude of the oscillations. The detector is usually a voltage amplitude sensor designed to produce an output when the amplitude falls below a specified level.
The nominal sensing range of the system is a function of the sensor diameter and the power which generates the electromagnetic field. Variations in the range can be large and it is not unusual to design for 100% margin due to the combined effects of manufacturing tolerances and temperature variations.
The thickness of the target has no significant effect on range if it is thicker than about one millimeter. The shape of the target and its metal content are the major influences on range. Sensing of nonferrous metals is more difficult and the range will be less for these objects. If the sensor must be imbedded in metal, it is usually shielded on all sides but one. This focuses the energy to the front of the sensor, but it also reduces the range of the detector compared to an unshielded sensor of the same size.
Many implementations of the basic proximity detector have been developed. They all consisted of an oscillator, either Colpitts or Hartley, operating at about 100 kHz, and some form of amplitude detector. Some emphasized sensitivity in an attempt to achieve a large change in output amplitude for the smallest targets. Others were micropower circuits designed to operate continuously. A few even combined the ideas and achieved modest success. The sensors included both shielded and unshielded inductors ranging from about 10 millimeters to about 25 millimeters in diameter.
The problem with all circuits was the basic conflict between realizing an oscillator that oscillates readily and reliably and yet exhibits a significant reduction in output in the presence of a minor disturbance (the coin). A stable oscillator experiences only a minor change in output when the field is disturbed, while a marginal oscillator experience changes, but may not regenerate when the disturbance is removed.
The best compromise that could be achieved was a Colpitts configuration biased for 20 microamperes continuous current, which exhibited about a 25% reduction in amplitude for the smallest coins. Unfortunately, temperature variations make this and other attempts virtually useless as reliable proximity detectors.
An alternative design used the sensing coil in a parallel tuned circuit which is driven periodically at a low frequency of about 30 Hz with a very short impulse, generating a decaying oscillatory response. The response is amplified and the number of the natural resonant frequency are counted. The number depends on the Q of the tuned circuit which is determined primarily by the coil.
The presence of metal objects again causes additional losses which reduce the Q and the number of cycles generated in response to an impulse. The output cycles then decrement a presettable counter which periodically restarts a “watchdog” timer as long as the required number of cycles are counted. In the presence of a coin, fewer cycles are counted and the watchdog times out, generating a detect signal. This design suffers from relatively small changes in Q for small coins, a deficiency which can be improved by longer counting intervals at the risk of missing some coins. The technique could be made more adaptive, but that would require either more circuitry or powering the normally quiescent controller to supply the intelligence.